Fix an issue that SwapChainDesc.DepthBufferFormat = TEX_FORMAT_R24G8_TYPELESS causing D3D12 invalid param error

[hzqst]

An invalid param error D3D12_CLEAR_VALUE::Format cannot be a typeless format. A fully qualified format must be supplied. Format = R24G8_TYPELESS. was thrown when ClearValue.Format set to DXGI_FORMAT_R24G8_TYPELESS.

We just leave the Desc.ClearValue.Format empty and GetClearFormat wil return a proper format for us.

//TextureD3D12Impl.cpp~L169
if (d3d12TexDesc.Flags & (D3D12_RESOURCE_FLAG_ALLOW_RENDER_TARGET | D3D12_RESOURCE_FLAG_ALLOW_DEPTH_STENCIL))
    {
        if (m_Desc.ClearValue.Format != TEX_FORMAT_UNKNOWN)
            ClearValue.Format = TexFormatToDXGI_Format(m_Desc.ClearValue.Format);
        else
        {
            DXGI_FORMAT Format = TexFormatToDXGI_Format(m_Desc.Format, m_Desc.BindFlags);
            ClearValue.Format  = GetClearFormat(Format, d3d12TexDesc.Flags);
        }